He had been accepted for fall 2019, had a busy summer and just forgot.
My daughter was getting him registered for fall 2021 classes in May and when she attempted to submit his class schedule, his account was labeled “never matriculated”.
She got on the phone with admissions, met with an officer the next day, had his deferment completed and his scholarship was also reinstated.
He returns in a week and enrolls in August.
Hopefully there is more to the story here, because even the BYU Admissions site has an FAQ about “forgot to defer”. It happens.